There has been known an inkjet printhead which generates thermal energy by a heater arranged inside its nozzle, forms ink bubbles near the heater by utilizing the thermal energy, and discharges ink from the nozzle by bubbling to print. FIG. 6 shows an example of a heater driving circuit in the inkjet printhead.
In order to print with such a printhead at a high speed, it is desirable to simultaneously drive as many heaters as possible and simultaneously discharge ink from as many nozzles as possible. However, the capacity of an electric power supply (power supply) of a printer is limited, and a current value which can be supplied at once is limited owing to a voltage drop caused by the resistance of a wiring line running from the power supply to the heater. From this, the printhead generally adopts time-division driving of driving a plurality of heaters by time division and discharging ink. In the time-division driving, the printhead comprises a plurality of heaters, the heaters (nozzles) are divided into a plurality of groups, each formed from a plurality of heaters arranged adjacent to each other. The heaters of the groups are driven by time division so that no more than two heaters are simultaneously driven in each group. The sum of currents flowing through heaters is suppressed, and no large electric power need be supplied at once. The operation of the driving circuit which drives heaters in this way will be explained with reference to FIG. 6.
As shown in FIG. 6, heaters 1101a1 to 1101mx and MOS transistors 1102a1 to 1102mx corresponding to the respective heaters are classified into groups a to m which accommodate the same numbers (x) of heaters and MOS transistors. In group a, a power supply line extending from a positive power supply pad 1104 is commonly connected to the heaters 1101a1 to 1101ax, and the respective MOS transistors 1102a1 to 1102ax are series-connected to the corresponding heaters 1101a1 to 1101ax between the power supply line and ground. The heaters 1101a1 to 1101ax are heated when a control circuit 1105 supplies a control signal to the gates of the corresponding MOS transistors 1102a1 to 1102ax to turn them on and a current flows from the power supply line via heaters series-connected to the transistors.
FIGS. 7A and 7B are timing charts showing timings at which the heaters of each group of the heater driving circuit shown in FIG. 6 are energized and driven. FIG. 7A shows a voltage applied to the base of each transistor, and FIG. 7B shows a current flowing through each heater in correspondence with the applying the base voltage.
Group a in FIG. 6 will be exemplified. Control signals VG1 to VGx are timing signals for driving the first to x-th heaters 1101a1 to 1101ax belonging to the group a. That is, VG1 to VGx represent the waveforms of signals input to the control terminals (bases) of the MOS transistors 1102a1 to 1102ax of the group a. When the control signals VG1 to VGx are at high level, they turn on corresponding MOS transistors 1102, and when the signals VG1 to VGx are at low level, turn them off. This also applies to the remaining groups b to m. In FIG. 7B, Ih1 to Ihx represent current values flowing through the respective heaters 1101a1 to 1101ax.
In this manner, heaters in each group are sequentially energized and driven by time division. The number of heaters energized and driven in the group can always be controlled to one or less, and no large current need be supplied to heaters at once.
FIG. 8 depicts a view showing an example of the layout of a heater substrate (substrate which forms a printhead) on which the heater driving circuit in FIG. 6 is formed. FIG. 8 illustrates the layout of power supply lines which are connected to groups a to m from the power supply pads 1104 shown in FIG. 6.
Power supply lines 1301a to 1301m and 1302a to 1302m are individually connected from the power supply pads 1104 to groups a to m. Since the number of heaters simultaneously driven in each group is controlled to one or less, as described above, a current value flowing through the wiring line divided for each group can always be kept equal to or smaller than a current flowing through one heater. Even when a plurality of heaters are simultaneously driven, a voltage drop amount on the line on the heater substrate can be kept constant. At the same time, even when a plurality of heaters are simultaneously driven, an energy amount applied to each heater can be kept almost constant.
In recent years, higher speeds and higher precision are requested of printers, and the printhead of the printer is equipped with many nozzles (heaters) at high density. In driving heater in the printhead, a larger number of heaters must be simultaneously driven at a high speed in terms of the printing speed.
The heater substrate is prepared by forming many heaters and their driving circuit on a single semiconductor substrate. Thus, the heater driving circuit is formed using a low-cost MOS semiconductor process which can fabricate smaller-size devices at higher density by a simpler manufacturing process in comparison with a conventional bipolar semiconductor process. Further, the heater substrate must be downsized because the cost must be reduced by increasing the number of heater substrates formed from one wafer.
As described above, if the number of simultaneously driven heaters is increased, the number of wiring lines corresponding to the number of simultaneously driven heaters must be laid out on the heater substrate. Along with this, the number of wiring lines increases, and when the area of each heater substrate is limited, the wiring resistance increases because the wiring region (width) per wiring line decreases. In addition, each wiring width decreases, and the resistance more greatly varies between wiring lines on the heater substrate. This problem also occurs in downsizing the heater substrate, increasing the wiring resistance and variations in resistance of the wirings. Since a heater and power supply line are series-connected to the power supply on the heater substrate, as described above, a voltage applied to each heater fluctuates at a higher ratio owing to increases in wiring resistance and variations in resistance of the wirings.
Excessively small energy applied to the heater makes ink discharge unstable, but excessively large energy degrades the heater durability. For high-quality printing, energy applied to the heater is desirably constant. However, if a voltage applied to the heater greatly fluctuates, the heater durability degrades or ink discharge becomes unstable.
In a case where a printhead has a plurality of heater substrates, since the wiring line is commonly connected to a plurality of heaters across the heater substrates, a voltage drop on the common wiring line changes at each head substrate, depending on the number of simultaneously driven heaters of each head substrate. In order to keep energy applied to each heater constant over the plurality of heater substrates upon variations in voltage drop, energy applied to the heaters of each heater substrate is adjusted by the voltage application time. However, the voltage drop on the common wiring line becomes larger with an increase in the number of simultaneously driven heaters. The voltage application time prolongs in-driving the heaters in accordance with the number of heater substrates, and it becomes difficult to drive the heaters at a high speed.
Japanese Patent Laid-Open No. 2001-191531 proposes a method which solves problems caused by variations in energy applied to the heaters. FIG. 9 is a circuit diagram showing a heater driving circuit disclosed in Japanese Patent Laid-Open No. 2001-191531. In this reference, heaters (R1 to Rn) are driven by a constant current by constant current sources (Tr14 to Tr(n+13)) and switching elements (Q1 to Qn) which are arranged for the heaters (R1 to Rn) corresponding to printing elements. This configuration can always drive heaters by a constant current regardless of variations in voltage drop outside the heater substrate along with an increase in the number of driven heaters.
In this case, constant current sources equal in number to printing elements are required, the area on the heater substrate greatly increases, and thus the cost of the heater substrate rises. In order to stabilize energy applied to the heater, output currents must be equal between a plurality of constant current sources. However, as the number of constant current sources increases, the output currents more greatly vary between the constant current sources. Especially when the number of heaters increases for a higher-speed, higher-precision printer, the number of constant current source circuits increases, and it becomes difficult to reduce variations in output current.